Crosshair drawing is broken when videoAr != displayAr

I'm not sure about that scale_factor, i don't know how to test that

with a "good enough" implementation

The preview window would change in width, when the video was being zoomed.
Possibly, because on windows, zooming only calls UpdateSize once, but on
linux, it gets called multiple, like 8 times.
Rounding fixed it.

I'm not entirely sure, that this is how it's supposed to be added.
This requires editing the already existing hotkey.json file (shows path
in log, if it exists).
Should this also be added to hotkey.cpp?

This reverts commit 3d525d9.
The default hotkey is not required. People can just add a custom one
from the hotkey settings if they want this.